Fuel senders, as they are commonly called, are actually transducers that convert the level of fuel in a fuel tank to an electric signal to proportionally drive a visually readable electric instrument, such as a bi-torque gauge. These fuel senders conventionally include a float carried on the end of a long pivoting arm suspended within the interior of the fuel supply tank, and this arm pivots a short wiper assembly having a ball contact slidably engaging the wires on a wound wire rheostat that varies in resistance in accordance with the position of the float and, hence, liquid level, however, not always in directly proportional fashion. The angular position of the pivoting float arm, more particularly the incremental angle of the arm, is not directly proportional to a vertical incremental change in float position and, hence, not directly proportional to liquid level. Moreover, in some cases the instrument itself is not proportional throughout its range and requires some compensation. The rheostat, and more particularly the wire conductive windings of the rheostat, provides a convenient location for compensating for the non-linearity in one or two, or both ways. The first is to vary the space between the wire turns and, more specifically, by increasing the wire turn spacing the ratio of resistance change to float arm angle change decreases, and conversely, it increases with more tightly wound turns. Another common way of varying linearity is to change the individual turn lengths by varying the shape of the support board on which the wire is wound. One common shape variation includes a tapered section in the form of a frusto-isosceles triangle.
There are five critical dimensions in the design of an appropriate fuel sender for a particular size fuel tank. These dimensions are tank depth, float arm radius, the depth of the axis of rotation of the float arm, the length of the frame that supports the rheostat in the tank, and the conductor wire lengths extending between terminals at the top of the sender and the rheostat. The latter four dimensions, however, are all controlled by the former, namely the depth of the tank. For example, in one exemplary fuel sender for a tank depth of 6 inches, the float arm radius is 2.31 inches, the pivotal axis of the rheostat is 3.19 inches, the length of the support for the rheostat is 5 inches, and the wire length of 5.25 inches. For a similar rheostat and a tank depth of 26 inches, the float arm radius is 13.86 inches, the rheostat pivot axis dimension is 13.19 inches, the length of the support frame for the rheostat is 15 inches, and the wire length is 15.25 inches.
It can be readily seen from this that if a manufacturer is to accommodate tank depths form 6 inches to 26 inches at 1/2 inch intervals, it would have to manufacture 41 different fuel senders. The tooling and manufacturing costs to manufacture this array of fuel senders is quite costly, and therefore, it has been found desirable to design a fuel sender in which the same parts can be used in making fuel senders having significantly varying critical dimensions.
It is a primary object of the present invention to ameliorate the problems noted above in the manufacture of fuel senders for different depth tanks.